Heretofore, a liquid crystal/polymer composite liquid crystal optical element equipped with an operation mode of transmission/scattering type has been known. The liquid crystal/polymer composite controls a light by a difference in refractive index between the polymer and the liquid or in the domain in the interior of the liquid crystal. Mainly, a voltage is applied to the facing electrodes to change the optical characteristics. It is also called a polymer dispersed liquid crystal element (PDLC) or simply a dispersed liquid crystal.
A transmission/scattering type liquid crystal optical element requires no polarizing plate in principle, as different from a TN or STN liquid crystal optical element. Accordingly, it basically provides a high light transmittance, and it is used, in combination with the scattering characteristics, for e.g. a light-controlling glass, an optical shutter and a laser equipment.
As basic technique of a liquid crystal optical element having a liquid crystal/polymer composite layer, the following may be mentioned. First, prior art reference 1 discloses formation of a liquid crystal optical element from a mixed liquid of a liquid crystal and a polymerizable liquid crystal. The mixed liquid is disposed in a liquid crystal cell in an aligned state and irradiated with ultraviolet rays to form a gel from the mixed liquid. In prior art reference 1 (U.S. Pat. No. 5,188,760), this gel is referred to as an anisotropic gel in particular.
Further, in the liquid crystal optical element as disclosed in prior art reference 1, the refractive index of the liquid crystal when no voltage is applied and the refractive index of the anisotropic gel (polymerized polymer) substantially agree with each other. Accordingly, an optical element highly transparent regardless of the direction of view can be obtained. Further, when a voltage is applied, the alignment of the liquid crystal changes due to the dielectric anisotropy of the liquid crystal, whereby the refractive index of the liquid crystal and the refractive index of the anisotropic gel become different from each other, and the optical element exhibits a scattering state.
Prior art reference 2 (WO 92/19695) discloses one comprising a chiral nematic liquid crystal having positive dielectric anisotropy and a very small amount of a polymer dispersed in the liquid crystal. Its basic structure is the same as in prior art reference 1. A transparent state is obtained under application of no voltage and a scattering state is obtained under application of a voltage. The liquid crystal optical element of prior art reference 2 is called PSCT (polymer stabilized cholesteric texture).
Further, prior art reference 3 (European Patent Publication 1154006 A1, (EXAMPLE 7)) discloses that a mixture of a polymer and a liquid crystal having negative dielectric anisotropy is sandwiched between vertical alignment films to form a liquid crystal/polymer composite by polymer phase separation.
A reverse mode dispersed liquid crystal optical element as disclosed in the above prior art references 1 to 3, i.e. an element which shows a transparent state when no voltage is applied, looks like a glass plate. Accordingly, it can be used as a highly transparent glass product with functions added. For example, it can be used as a light-controlling glass, a head-up display, a display panel of game machines and a public display device.
For such uses, the surface of a liquid crystal optical element is not protected and is exposed in many cases, so as to obtain high transparency. However, in such a sate, it is likely that a person directly touches or an object bumps against the surface of the liquid crystal optical element. In such a case, on the surface, particularly on the portion to which an impact is applied, of the liquid crystal optical element, white turbidity which will not become transparent again may occur in some cases.
The cause of this white turbidity is considered to be because the structure of a polymer (such as a polymerized product of e.g. a monomer) in the interior of the liquid crystal optical element is destructed and deformed. Further, it is considered to be because not only the shape of the polymer is partially deformed but also the alignment of the liquid crystal around the periphery thereof is changed.
The portion with such white turbidity will not recover to the original state even when a driving voltage sufficient to change the optical state is applied to the liquid crystal optical element. So-called reversible stable optical state is impaired. Thus, at the portion with white turbidity, the display function is lost, and the originally desired display operation can not be carried out.
Further, there will be the above problem of white turbidity also when a liquid crystal optical element is produced or when the liquid crystal optical element is assembled into e.g. a display device. For example, an impact is applied to a liquid crystal cell in a production process by some external factor in some cases. Then, white turbidity as mentioned above occurs, which leads to a decrease in yield in the entire process for producing a liquid crystal optical element.
Under these circumstances, the present invention has been made to overcome the above problems, and its object is to provide a liquid crystal optical element excellent in impact resistance, and a process for producing it.